Transfer hydrogenation reduction of ketones, aldehydes and imines using chelated iridium(III) N-heterocyclic bis-carbene complexes

• Published in: Polyhedron Vol. 23; no. 17; pp. 2857 - 2872
• Main Authors: Miecznikowski, John R., Crabtree, Robert H.
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 11.11.2004; Elsevier
• Subjects: Aldehydes; Chemistry; Chemistry, Inorganic & Nuclear; Crystallography; Homogenous catalysis; Imines; Ketones; N-Heterocyclic carbenes; Physical Sciences; Science & Technology; Transfer hydrogenation; Transfer hydrogenation; Ketones; Imines; Aldehydes; Homogenous catalysis; N-Heterocyclic carbenes; COORDINATION CHEMISTRY; homogenous catalysis; ASYMMETRIC TRANSFER HYDROGENATION; LIGAND-BINDING; CATALYTIC ENANTIOSELECTIVE HYDROGENATION; transfer hydrogenation; imines; OLEFIN METATHESIS; CHIRAL TITANOCENE CATALYST; ketones; STEREOSELECTIVE HYDROGENATION; EFFICIENT CATALYSTS; aldehydes; RUTHENIUM COMPLEXES; IMIDAZOLIUM SALT; N-heterocyclic carbenes
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2004.07.001

We have developed a series of iridium bis-imidazole and bis-triazole precatalysts for the transfer hydrogenation reduction of ketones, aldehydes, and imines in 2-propanol. The choice of linker group is important; the best results were obtained with methylene and neopentanediyl linkers. A series of imidazolium and triazolium chelate ligand precursors are metalated with [Ir(coe) 2Cl] 2 (coe = cyclooctene) or [Ir(cod)Cl] 2 (cod = 1,5-cyclooctadiene) to give iridium(III) N-heterocyclic carbene (NHC) chelates [(NHC)(linker)(NHC)IrI 2(OAc)]. The triazolium salts metalate more easily and give less electron donating NHCs as shown by IR spectroscopy on analogous iridium(I) carbonyl derivatives. The iridium(III) complexes show activity for the title catalytic reactions in 2-propanol that is very variable, depending on the nature of the linker and terminal N-substituents. Selective reduction of aldehydes over ketones becomes possible with K 2CO 3, an alternative base promoter to the usual KOH. TOFs of up to 1500/h were achieved.